1. Field of the Invention
The present invention relates to an optical waveguide element module, and more particularly, to an optical waveguide element module including a connector to which an external signal line for inputting a modulation signal to a modulation electrode of an optical waveguide element is connected and a relay line formed on a relay substrate so as to connect the connector to the modulation electrode.
2. Description of the Related Art
Hitherto, an optical waveguide element such as an optical modulator in which an optical waveguide is formed on a substrate having an electro-optical effect has been widely used as the means for controlling optical waves in the field of optical communications and optical measurements.
The optical waveguide element is provided with a modulation electrode modulating optical waves propagating in the optical waveguide and a modulation signal is input to the modulation electrode through a connector to which an external signal line is connected. Accordingly, in order to efficiently input the modulation signal to the modulation electrode from the external signal line, it is necessary to match the impedance of the modulation electrode with the impedance of the external signal line and to prevent reflection of the modulation signal in a transmission line.
FIG. 1 shows an example of an optical waveguide element module. The optical waveguide element includes a substrate 1 formed of a material having an electro-optical effect, an optical waveguide 2 formed in the substrate, and a modulation electrode 3 (in which the modulation electrode includes a signal electrode and a ground electrode but only the signal electrode is shown in FIG. 1 for the purpose of simplification) modulating optical waves propagating in the optical waveguide 2. An input optical fiber 4 for inputting the optical waves and an output optical fiber 5 for outputting the modulated optical waves are connected to the optical waveguide element. The optical waveguide element is air-tightly housed in a case 9 to form an optical waveguide element module. A modulation signal is applied to the modulation electrode 3 of the optical waveguide element through a connector 8 from a driver 6 disposed outside the optical waveguide element module.
The impedance from the driver 6 to the connector 8 is generally set to 50Ω. When the impedance of the modulation electrode of the optical waveguide element is 40Ω, the modulation signal is reflected from between the connector 8 and the modulation electrode 3 due to the impedance mismatch between the transmission lines as described above. In order to solve this problem, a relay substrate 7 is disposed between the connector 8 and the optical waveguide element, a resistor 11 (a 10Ω resistor in this case) is disposed in the relay substrate 7 as shown in FIG. 2, and the impedance closer to the optical waveguide element than the connector 8 is set to about 50Ω.
This method of adjusting the impedance is effective for suppressing the reflection of microwaves which are the modulation signal, but causes a problem in that the microwaves are attenuated by the resistor 11 and it is thus difficult to effectively apply the modulation signal to the optical waveguide element.
On the other hand, it has been proposed that the relay substrate 7 mounted with a filter circuit or the like is used to make the electro-optical conversion response characteristic of the optical waveguide element be a flat frequency characteristic from a low-frequency area to a high-frequency area as shown in FIG. 3 (see PTL 1).